This invention relates to tools for comminuting a hard mineral formation such as rock, coal, pavement, etc. Such tools are generally characterized by hard metal inserts which are usually formed of a sintered metal carbide, for example, tungsten carbide, which inserts engage the mineral matter and provide a longer wear life than hardened steel. These inserts or compacts are normally press fitted into sockets bored into a hard steel tool head. The sockets are located according to a predetermined pattern. Typical examples of such tools include percussion drill bits, multi-rotary cutter rock drills, coal picks, and pavement breaking picks. Reference may be had to the patent to Klima U.S. Pat. No. 4,098,358, for details of one form of a rotary cutter rock drill in which the present invention is useful. Other forms of tools utilizing inserts or compacts in the working surface thereof are well known.
A common problem in the drilling of oil well and gas well holes with carbide insert containing multi-rotary cutter drill bits is the failure of the drill bit due to one or more of the tungsten carbide inserts coming loose or breaking out of its socket during the drilling process. This event usually results in several more of the tungsten carbide inserts breaking off the drill bit. The loose carbide insert is extremely hard and beyond the ability of the drill to disintegrate. Such a loosened insert is generally driven around the bottom of the drill hole as the drill rotates and until it is caught between rows of nibs which are rotating at the end of the bit. This frequently results in cracking or other destruction of several more of the carbide inserts.
Additionally, it has been found that the steel body of the rotary cutter as it works wears off a case hardened skin. This skin may be at a hardness of between 42-62 Rockwell C. The physical wear of the surface containing the tungsten carbide insert causes a decrease in the necessary retaining length or depth of the socket holding the shank of the carbide nib or insert. The inserts are normally pressed into the steel bodies with an interference "press fit" and no other retention means are used. The vibration and pounding action against the insert also tends to loosen and dislodge the insert from its socket in the body. Side loading due to the geometry of the insert may also accelerate loosening of the inserts from the rotary cutter bodies in rock drills.
With percussion drill bits, the problem is not as pronounced because most of the forces on the inserts are along the axis of the insert and the axis of the hole or socket holding the insert. In the manufacture of percussion bits and multi-rotary cutter bits, considerable manufacturing time is spent in sorting and measuring the reamed hole diameters in the bit bodies, and then matching the hole with the sized carbide inserts in order to achieve the correct interference fit. When inserts fall out of percussion type bits, it is usually because the matching of the insert to the socket has been badly carried out. Loss of an insert is less likely to be due to side loading while cutting through rock with a percussion type drill bit.
There has been provided a new locking device for holding the carbide insert into the working head of a tool for comminuting a hard mineral formation. Utilization of the structures of the present invention does not require a basic change in the design of the bits.